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The House is expected to pass today, by a substantial margin, legislation that would loosen President Bush's restrictions on human embryonic stem cell research -- a bill identical to the one it passed in 2005.

Next month, the Senate is expected to do the same, as it did last year.

And all indications are that soon after that, Bush will, for the second time, veto the bill.

But the final outcome of this year's emotional fight over the science and ethics of stem cell research is not as predictable as it may seem, said scientists and congressional strategists on both sides of the issue.

Opponents of the research say they have never been stronger, not only because of the ongoing support of the president but also because several recent studies have suggested that non-embryonic cells have significant medical promise that may rival that of embryonic cells.

Proponents, however, have a new ace up their sleeve as well: the political shifts wrought by voters in November. With stem-cell-friendly Democrats in the majority for the first time since the cells were discovered in 1998, supporters of the research will be able to work Congress's complex rules in their favor.

"This is not a 'one bill and you're out,' but a two-year time frame with potentially multiple legislative possibilities," said a research supporter involved in Capitol Hill strategizing, speaking on the condition of anonymity because he is not authorized to comment publicly.

"They can stretch it out," the strategist said, so Bush and his backers in Congress must keep saying no to research that the public says it strongly supports. The research advocates "can make it as painful as possible."

But if it comes to war, members of Congress and aides from both parties said, supporters have many options not available to them last year. They range from subtle moves that could enhance the odds of overriding a veto to heavy-handed tactics such as attaching the bill to must-pass budget legislation.

"I'm confident we'll have a veto-proof bill this time," said Sen. Arlen Specter (R-Pa.), one of three high-powered Republican supporters of stem cell research who attended the Tuesday briefing. In an odd Washington moment, all three seemed briefly grateful that the Democrats were back in charge.

Sen. Tom Harkin (D-Iowa), who has led the stem cell charge in the Senate along with Specter, agreed. "One way or another, we're going to get this done this year," he said yesterday in a meeting with Washington Post editors and reporters.